EUDC is built-in to Emacs, and its main functions are autoloaded.
After installing EUDC you will find (the next time you launch Emacs) a
Directory Search submenu in the ‘Tools’ menu that will
give you access to EUDC.
You may also find it useful to add the following to your .emacs initialization file to add a shortcut for email address expansion in email composition buffers (see Inline Query Expansion)
(with-eval-after-load "message" (define-key message-mode-map [(control ?c) (tab)] 'eudc-expand-try-all)) (with-eval-after-load "sendmail" (define-key mail-mode-map [(control ?c) (tab)] 'eudc-expand-try-all))